The inventional system is based on a system for controlling a vehicle body level to a presettable level. Specifically the system according to the present invention controls a vehicle body level to a presettable level with a selectable time delay of the control operations and with at least one state of operation which is adjustable at certain operating conditions of the vehicle in order to minimize the control operations in loading and/or unloading of the vehicle.
Such level control systems are known, e.g., from the article "Elektronisch geregelte Luftfederung fur Nutzfahrzeuge" [Electronically Controlled Air Suspension for Trucks], VDI-Berichte No. 687, 1988, pages 161 through 181. This article describes an electronically controlled air suspension which as an adaptive system comprises two system parts, level control and level adjustment. The closed loop is formed by a displacement sensor measuring the distance between axle and body, by an electronic controller for signal processing, by a solenoid valve block for inflating and deflating the air suspension bellows, by the air suspension bellows themselves supporting the load between axle and body, and by the body with the payload and the corresponding force of the weight. Such electronic air suspension serves to keep the vehicle height at a preset level, independently of the state of load. As mentioned above, this takes place by means of displacement sensors arranged on the axles, the signals of which sensors are compared to set values by an electronic controller. Exceeding a preset deviation causes appropriate actuators to be activated by the controller in such a way that a pressure increase or pressure reduction is effected in the air suspension bellows, returning the vehicle height again to the set level. This function is effective both at standstill and travel of the vehicle. Level control assumes a special function in loading at standstill. Remote control provides here the option to gradually approach a level ranging within physical limits and store it as set level. A typical application of this function is the loading operation at a loading dock, where a best possible agreement of dock level and vehicle level simplifies the loading operation considerably. In loading the vehicle, when the vehicle level drops due to the increasing load it is then raised again, and a rise of the vehicle level in unloading is avoided as well. Unloading and loading is problematic when heavy industrial trucks enter the vehicle, such as forklift, which cause a considerable variation in level already by their dead load. The level correction carried out by the air suspension at each load variation results then within a relatively short time in the exhaustion of the compressed air supply, whereby a further level correction is then no longer possible. A remedy is available here merely by starting the vehicle engine and recharging the pneumatic system. In the above article, the driver has the option of selecting on the operating panel of the air suspension system the respective "loading" or "unloading" operation for the vehicle. In loading the vehicle, e.g., only the raising of the body is then settled. Any interim relief goes unallowed for. From the above article it follows also to install, notably in travel operation, a dead band around the set level, in order to preclude that any slight deviation of the actual level from the set level will trigger a control operation. Furthermore, it is described here to provide a delay time between leaving the dead band and the start of the control action.
The objective of the present invention is optimizing the level control in the case of loading and unloading.